This research proposal focuses on elucidating the nature of integrin-ligand interactions for the predominant cell surface adhesion molecule displayed on human osteoclasts, the alpha(v) beta(3) integrin (vitronectin receptor). The investigators propose to map the ligand binding surface of alpha(v) beta(3) using an integrated approach, which combines chemistry of photoaffinity scanning, followed by protein digest mapping of the ligand integrin receptor photoconjugates, and structure-function analysis of the receptor utilizing the molecular biology of site-directed mutagenesis. In the proposed studies, the "contact sites" between ligand the cloned human alpha(v) beta(3) receptor will be examined using a novel set of "tagged" peptide analogs, which incorporate a photoreactive group and a photoreactive iodine or biotinylated derivative. An alpha(v)beta(3) affinity column will be used to select high affinity ligands from a large mixture present in synthetic combinatorial libraries. Putative contact sites will be confirmed by making substitutions in the receptor by site-directed mutagenesis. The investigators hope to develop an experimentally-derived model of the alpha(v)beta(3) integrin ligand bimolecular complex, which can then serve as a template for rational design of alpha(v)beta(3) antagonists for the treatment of osteoporosis and other disorders of bone metabolism.